Video signals can be represented as either composite video or component video. Using frequency interleaving, composite video contains luminance (e.g., Y) and chrominance (e.g., U, V) information in a single signal. In component video, the color “component” signals (e.g., red, green and blue) are carried on separate wires. Video synchronization can be signaled by either dedicated horizontal synchronization (Hsync) and vertical synchronization (Vsync) signals, as is normally the case for PC graphics, or both types of synchronization signals can be combined into a single “composite synchronization” signal. Composite video carries an embedded composite synchronization signal. Component video may embed a composite synchronization on at least one of its signal components, or, alternatively, may carry a composite synchronization or Hsync and Vsync signals on additional separate wires. The characteristics of a video format include the number of rows and columns per field, the number of fields per frame (e.g., 1 for progressive-scan, 2 for interlaced-scan video) and the number of frames per second.
Presently, three systems for composite video exist worldwide. The systems are the National Television Standard Committee (NTSC) format, the Phase Alternating Line (PAL) and the Sequentiel Couleur A Memoire (SECAM) format. Within each system, several variations or sub-standards exist. Component video can be represented equivalently in one of several color spaces (e.g., RGB, YCbCr). Component video in any of these color spaces can possibly include a “composite synchronization” signal inserted on a single component (e.g., inserted on the green or luma (Y) channel, referred to as sync-on-green or sync-on-Y). For standard definition TV, component video can have the same video timing characteristics as its corresponding NTSC, PAL or SECAM composite formats. Composite video is only defined for standard-definition video signals. For PC graphics and high-definition TV, only a component video representation exists. Furthermore, PC graphics formats are normally always carried in the RGB color space and can have different timing characteristics depending on resolution and screen refresh rate (e.g., VGA, SVGA, XGA SXGA @ 60, 75 or 85 Hz refresh rates). On the other hand, digital television video (DTV) formats typically utilize component video signals in the YCbCr domain not the RGB domain.
Traditionally, only a limited number of video display formats have been in use for standard-definition television (SDTV). Recently, the introduction of high-definition television (HDTV) and the convergence of personal computers and television applications have increased the number of display formats that may need to be supported by consumer video end equipment such as set-top boxes (STB), Digital Video Disc Players (DVD), Digital-TV (DTV) receivers, Personal Computer television (PCTV) end equipment or high-end (projection) TV sets. When only a limited number of video display formats need to be supported, the “display timing generator” sub-block, which generates the required output video synchronization that accompanies the analog component video signal, can be hard-coded within the video “source” equipment (e.g., DVD player, STB, computer graphics cards). This video synchronization accompanies the video signal and allows the video “sink” equipment (e.g., TV set, projector, PC monitor) to properly display the video content.
As previously discussed, the video synchronization signals in TV systems are typically not carried separately from the video signal itself, as is commonly the case in the PC graphics environment. Instead such synchronization is embedded within the analog component video signal as a “composite synchronization” signal. Furthermore for broadcasting, the bandwidth of the overall video signal, that includes the embedded composite synchronization signal, needs to be limited. Therefore, the shape of the composite sync should be controlled, in particular its rise and fall time. Such “synchronization shaping” presents an additional requirement not found in PC graphics systems, which utilize binary (non-bandwidth limited) Hsync/Vsync signals carried on dedicated wires.